I have replaced the down stream (after Cat) O2 sensor and I am still getting Codes P0137 & P0141 ... I seen info stating Bank1 Sensor2 is "before" Cat.... is it Before or After the Cat ?...on a 1997 Eldorado ETC

P0137 = 02 sensor circuit low voltage (Bank1 sensor2)
P0141 = 02 sensor heater circuit malfunction (Bank1 sensor2)

Thanks in advance.